1. Field of the Invention
This invention relates generally to voice switched telephone amplifier circuits of the type especially used in microphone and loudspeaker telephone sets or speakerphone and loudspeaker telephone sets, sometimes known as "hand-free" telephones.
2. Description of the Prior Art
In a telephone set using a loudspeaker, it is necessary (i) to provide a constant signal amplification within the telephone set for supplying increase gain to the receive signal or to both the transmit and the receive signal and (ii) to provide a variable attenuation for the minimization or complete prevention of undesirable transfer of energy of speech signals between the receiving loudspeaker and the transmitting microphone of the telephone set. This is generally obtained by inserting in the transmitted speech circuit and in the listening circuit a constant gain amplifier and a variable loss attenuator and by controlling the attenuators in opposite directions by the detected received signal. When the detected received signal increases, the speech circuit attenuation is made to decrease and the listening circuit attenuation is made to increase.
Adding transmit and receive gain in a telephone set causes the sidetone signal which is heard in the loudspeaker to increase by the sum of the transmit and receive gains. If the sidetone signal increases to an objectionable level, it becomes ennoying to the user who has a tendency to lower his voice thus counteracting the purpose of the additional auxiliary amplification. U.S. Pat. No. 3,963,876 issued June 15, 1976 to Roger Edward HOLTZ et al. disclosed a voice switched telephone set comprising a transmit amplifier in the transmit circuit, a receive amplifier in the receive circuit, means for forming the sum of the in-phase transmit and receive signals in the hybrid netword of the telephon set and means for controlling the receive amplifier by this sum signal whereby the amplification of the receive amplifier is attenuated thus providing sidetone control and gain control.